This makes me thing that this will be a command injection vulnerability. We know that all the passwords are stored in: /etc/natas_webpass/ and we know that grep will search an input file for a given string pattern. We can also check that input is not sanitized by inputing "--help" into the search box and this should output the grep help screen:

So our command to output the password should look something like:

grep . /etc/natas_webpass/natas10

However, we only have control of whats placed in the $key variable. So our command should look like:

grep -i . /etc/natas_webpass/natas10 dictionary.txt

This isn't an issue because -i just says the search string is case insensitive and the dictionary.txt will just be output but out result will be first. after inputing this, I get the following result:

And as you can see, the first output is the password we are looking for: